It really is quite simple! Buy your SSD (I purchased the Crucial M4 as it seems to have the best reviews) and get a mounting kit depending on where you want to install the disk.
I opted to install it in the second DVD bay slot as this leaves your 4 HD bays free for other disks. All I needed was a mounting kit, and I got the: Akasa SSD & HDD adapter – fits 3.5″ or 2.5″ HDD into a 5.25″ PC bay. Just make sure your Mac Pro has a spare SATA cable in the DVD bay – just pull out the DVD bay towards you to check (from the side with the Mac open – not the front of the Mac!)
Once you’ve installed the drive onto the mount and then into the Mac, you’re done! Easy huh?
If you’re doing a clean Lion install check out my guide on how to do that too.